Dental Materials: Alginate Mixing and Setting
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Questions and Answers

What happens if the water/powder ratio is altered during alginate mixing?

  • It speeds up the impression setting time drastically.
  • It increases the strength of the impression.
  • It has no effect on the outcome.
  • It alters the consistency and setting times. (correct)

    • Which is the recommended mixing time for fast-set alginates?

  • 90 seconds
  • 60 seconds
  • 45 seconds (correct)
  • 30 seconds

    • How long should a typical alginate impression be left in place for optimal tear strength and elastic recovery?

  • 1 to 2 minutes
  • 2 to 3 minutes (correct)
  • 5 minutes
  • 3 to 4 minutes

    • What is the effect of using water that is cooler than 18°C during alginate mixing?

    <p>It may compromise the setting reaction.</p> Signup and view all the answers

    Which method is recommended to lengthen the setting time of alginate?

    <p>Reducing the temperature of the water</p> Signup and view all the answers

    What is the typical elasticity percentage for most alginates?

    <p>14%</p> Signup and view all the answers

    What is the risk of undermixing or overmixing alginate?

    <p>It diminishes the strength of the set impression.</p> Signup and view all the answers

    What is a significant benefit of providing adequate time for recovery after taking an alginate impression?

    <p>Increases the dimensional stability of the impression.</p> Signup and view all the answers

    What is the phenomenon called when the viscosity of impression materials decreases with increased shear force?

    <p>Shear thinning</p> Signup and view all the answers

    What happens to the viscosity of polyether impression materials when the shearing force is removed?

    <p>It increases immediately</p> Signup and view all the answers

    What primarily influences the shear-thinning properties of polyether impression materials?

    <p>Microcrystalline triglyceride network</p> Signup and view all the answers

    How does temperature affect the working and setting times of elastomeric impression materials?

    <p>Decreases working and setting times</p> Signup and view all the answers

    What causes dimensional change in impression materials during setting?

    <p>Cross-linking and rearrangement of bonds</p> Signup and view all the answers

    What term describes the behavior of polyether impression materials where they transition sharply into the setting phase?

    <p>Snap-set</p> Signup and view all the answers

    What can cause an impression to expand after setting has occurred?

    <p>Water sorption</p> Signup and view all the answers

    What is likely to happen if an impression material does not recover elastically after being removed from undercuts?

    <p>Creep will occur</p> Signup and view all the answers

    What is the compressive strength range of alginate impressions?

    <p>0.5 to 0.9 MPa</p> Signup and view all the answers

    Why is tear strength considered more important than compressive strength in alginates?

    <p>It affects the likelihood of tearing during removal.</p> Signup and view all the answers

    What must be done to an alginate impression before preparing a gypsum model?

    <p>It must be disinfected and rinsed to remove impurities.</p> Signup and view all the answers

    What happens to alginate impressions left out for 30 minutes?

    <p>They lose water and can become inaccurate.</p> Signup and view all the answers

    How should alginate impressions be stored if not used immediately?

    <p>Stored in 100% relative humidity.</p> Signup and view all the answers

    What is the primary risk associated with viruses in dental impressions?

    <p>Potential transmission to dental personnel.</p> Signup and view all the answers

    What is a common method for disinfecting alginate impressions?

    <p>Immersion in disinfectant or spraying.</p> Signup and view all the answers

    What is the effect of saliva and blood on gypsum setting?

    <p>They interfere with the setting of gypsum.</p> Signup and view all the answers

    What is the approximate contraction percentage of addition silicones?

    <p>−0.15%</p> Signup and view all the answers

    Which elastomeric impression material has the best elastic recovery during removal from the mouth?

    <p>Addition silicones</p> Signup and view all the answers

    How does the strain in compression under a stress of 0.1 MPa relate to the material's flexibility?

    <p>It indicates the material’s stiffness.</p> Signup and view all the answers

    Which statement about the hardness of addition silicones is correct?

    <p>Hardness does not change significantly with time.</p> Signup and view all the answers

    Which material is considered to be the least flexible?

    <p>Polyethers</p> Signup and view all the answers

    What aspect of a material does tear strength indicate?

    <p>Ability to withstand tearing</p> Signup and view all the answers

    How can low flexibility and high hardness in impression materials be clinically compensated?

    <p>By producing more space between the tray and the teeth</p> Signup and view all the answers

    What technique is used to assess the wettability of elastomeric impression materials?

    <p>Tensiometry</p> Signup and view all the answers

    What is the primary factor that affects the accuracy of dental impressions?

    <p>Viscosity of the impression material</p> Signup and view all the answers

    Which impression material produces slightly better detail even under wet conditions?

    <p>Polyether</p> Signup and view all the answers

    What technique can compensate for dimensional changes on setting of elastomeric impressions?

    <p>Double-impression technique</p> Signup and view all the answers

    What is the effect of inadequate mixing of impression materials?

    <p>Inaccurate impressions due to polymerization issues</p> Signup and view all the answers

    What is the recommended handling time for elastomeric impression materials?

    <p>Short, as viscosity increases rapidly</p> Signup and view all the answers

    Which method produces fewer bubbles in impression materials?

    <p>Auto mixing</p> Signup and view all the answers

    What happens to the accuracy of second pours of gypsum products into elastomeric impressions?

    <p>They are less accurate than first pours</p> Signup and view all the answers

    How does the recovery period affect elastomeric impressions post-removal?

    <p>It can increase accuracy during recovery</p> Signup and view all the answers

    Study Notes

    Proportioning and Mixing

    • Consistency and setting times of alginate are impacted by water-powder ratio.
    • Regular alginate mixing time: 1 minute, fast-set alginate mixing time: 45 seconds.
    • Over-mixing and under-mixing negatively affect the strength of the impression.
    • Working time: 45 seconds for fast-set (30-75 seconds before setting) and 60 seconds for regular-set (2-3.5 minutes before setting).
    • Use mechanical mixing devices or mix vigorously in a flexible rubber bowl with an alginate spatula.
    • Color change is a pH-related change of a dye.

    Setting Time

    • Ranges from 1 to 5 minutes (at least 15 seconds longer than the stated working time).
    • Lowering water temperature is preferable to reducing powder to water ratio to lengthen setting time.
    • Reducing powder to water ratio decreases strength and accuracy of the alginate.
    • The setting reaction is a chemical reaction that doubles in rate with a 10°C increase in temperature.
    • Water temperature between 18°C and 24°C is recommended.

    Elastic Recovery

    • Alginate impressions compress about 10% in undercut areas during removal.
    • Permanent deformation is time-dependent.
    • Leave the impression in place for 2-3 minutes to increase tear strength and elastic recovery.

    Clinical Considerations

    • A reasonable bulk of alginate between the tray and teeth, appropriate retention, and rapid removal from the mouth are necessary for a good impression.
    • Disinfect the impression and produce a gypsum model to allow for recovery.

    Flexibility

    • Alginates have a typical flexibility of 14%.
    • Hard-set materials have flexibility ranging from 5% to 8%.
    • Flexibility is essential for easy removal of the impression.

    Strength

    • Compressive strength: ranges from 0.5 to 0.9 MPa.
    • Tear strength: ranges from 0.4 to 0.7 kN/m; more important than compressive strength.
    • Tear strength is a measure of the force/thickness ratio needed to initiate and continue tearing.
    • Tearing is more likely in thin areas of an impression and decreases with faster removal.
    • Both compressive and tear strengths are time-dependent properties.

    Compatibility with Gypsum

    • Rinse the impression with cold water to remove saliva and blood, then disinfect.
    • Remove surface water before preparing a gypsum model.
    • Saliva and blood interfere with gypsum setting, and free water dilutes the gypsum model.
    • If the alginate impression is stored for longer than 30 minutes, rinse with cool water to remove exudate and wrap in a moist paper towel sealed in a plastic bag.

    Dimensional Stability

    • Alginate impressions lose water by evaporation and shrink when exposed to air.
    • Impressions left out for as short as 30 minutes can become inaccurate.
    • Pour the model material into the alginate impression as soon as possible for optimal accuracy.
    • If model preparation is delayed, store impressions in 100% relative humidity in a plastic bag or damp paper towel.

    Disinfection

    • Viruses can be transferred to gypsum models and present a risk.
    • Spraying is the most common disinfection method, but immersion in disinfectant is also effective.

    Shear Thinning

    • Shear force affects viscosity of poly ether and addition silicone impression materials.
    • Shear thinning (or pseudo-plasticity) reduces viscosity of the unset material with increasing outside force or shearing speed.
    • Viscosity increases when the force is discontinued.
    • This property is important for monophase impression materials.
    • In polyether, shear thinning is influenced by triglyceride crystals which align when the material is sheared.
    • The triglyceride network keeps the polyether viscous in the tray but allows it to flow under pressure.
    • Cooling increases viscosity; bring pastes to room temperature before use.

    Working and Setting Times

    • Working and setting times for elastomeric impression materials decrease as viscosity increases.
    • Polyethers have a defined working time with a sharp transition to setting, known as snap-set.
    • Working and setting times are shortened by increased temperature and humidity.

    Dimensional Change on Setting

    • Elastomeric impression materials undergo dimensional change on setting.
    • Contraction during setting is due to cross-linking and bond rearrangement.
    • Expansion can occur if water sorption takes place.
    • Distortion can occur if the material is seated after partial setting.
    • Creep can occur if the material does not recover elastically after removal from undercuts.

    Mechanical Properties

    • Addition silicone and polyether impression materials undergo shrinkage due to polymerization.
    • Addition silicones have the smallest change (~0.15%) followed by polyethers (~0.2%).
    • Contraction is low due to minimal byproduct loss.
    • Most shrinkage occurs in the first hour after removal.
    • Prepare models promptly for maximum accuracy.
    • Elastomeric impression materials are more stable in air than hydrocolloid products.

    Elastic Recovery

    • Addition silicones have the best elastic recovery followed by polyethers.

    Strain in Compression

    • Strain in compression under 0.1 MPa stress measures material flexibility.
    • Polyethers are the stiffest, followed by addition silicones.

    Flow

    • Flow is determined 15 minutes after applying a 1 N load to a 1-hour-old cylindrical specimen.
    • Silicones and polyethers have low flow values.

    Hardness

    • Hardness increases from low to high consistency.
    • Addition silicones maintain relatively consistent hardness over time, while polyether hardness increases with time.

    Tear Strength

    • Hardness and strain in compression influence the force needed for removal.
    • Low flexibility and high hardness can be compensated by increasing impression material space.
    • Tear strength indicates the material's ability to resist tearing in thin areas and margins of periodontally involved teeth.
    • As consistency increases, tear strength slightly increases.
    • Polysulfide is the most flexible, and polyether is the least flexible.

    Wettability and Hydrophilization

    • Wettability is measured by advancing contact angle of water on the set impression material.
    • Tradition addition silicone is less wettable than polyether.
    • Wettability and hydrophilization affect detail reproduction and bubble formation when pouring gypsum.
    • Most impression materials produce acceptable detail under wet and dry conditions.
    • Polyethers produce slightly better detail than addition silicones and are unaffected by moisture.

    Relationship of Properties and Clinical Application

    • Accuracy, detail recording, handling ease, and setting characteristics are crucial in dental impressions.
    • Placement timing is critical as viscosity increases rapidly with time.
    • Thorough mixing prevents incomplete polymerization and inconsistent setting.
    • Auto mixing and mechanical mixing systems reduce bubbles and improve accuracy.
    • Manufacturers recommend leaving the impression in the mouth for a minimum time.
    • Dimensional changes can be compensated for with double-impression or putty-wash techniques.
    • A custom-made tray enhances accuracy with monophase and simultaneous dual-viscosity techniques.
    • Viscosity is the most important factor for minimizing bubbles and maximizing detail.
    • The amount of deformation and removal time can affect impression accuracy.
    • Second gypsum pours produce less accurate dies than the first.
    • Elastomeric impressions continue to recover for a period after removal, increasing accuracy over time.

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    Description

    This quiz covers the essential aspects of mixing and setting times for alginate in dental practices. Understand the impact of water-powder ratios, standard mixing times, and the importance of maintaining the right consistency. Additionally, explore how temperature affects the setting reaction for more accurate impressions.

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